1. Field of the Invention
The present invention relates to biodegradable netting, composites made with the biodegradable netting, and methods for making the same.
2. Background Art
The continuous extrusion of plastic netting started in the 1950's with the process described in U.S. Pat. No. 2,919,467 to Mercer. Extruded netting is netting in which the strands are extruded from a die, the joints therebetween being formed either within the die or immediately outside the die. A variety of configurations are known, such as square, diamond, twill, etc. Some of the more common materials used to prepare extruded netting are polypropylene, polyethylene (including very low, low, medium, high, linear grades, and ethylene copolymers), nylon, polybutylene, and blends thereof.
Currently, the extrusion process of choice for manufacturing plastic nets is one in which individual plastic strands are extruded in an interconnecting network to provide the net-like structure. Methods for practicing this technique are well known. For instance, U.S. Pat. No. 3,700,521; U.S. Pat. No. 3,767,353; U.S. Pat. No. 3,723,218; U.S. Pat. No. 4,123,491; U.S. Pat. No. 4,152,479 and U.S. Pat. No. 4,190,692 show apparatus and method for making nets by the continuous extrusion of individual plastic strands. The disclosures of the above-mentioned issued patents are incorporated by reference into the present application.
These nets have found a number of uses in commerce. For example, these nets have found use as erosion control netting, packaging netting, such as for onion and turkey bags, agricultural netting, such as turf netting, turf wrap, hay bail and netting for industrial, filtration and home furnishings applications.
Netting has also found use in certain composites. In such composites the netting is laminated to one or more fabric overlays. Chief among such uses and composites are fabrics for disposable diapers, incontinent briefs, training pants, bandages, dressings, diaper holders and liners and feminine hygiene garments, medical gowns, medical drapes, mattress pads, blankets, sheets, clothing, consumer wipes and other like products, such as building and construction composites.
Since netting materials often find their way into the environment, either through their implanting as a result of their intended use, or as waste, it has become desirable to provide netting which is biodegradable. A biodegradable plastic material is defined as plastic material that degrades as a result of the action of naturally occurring micro-organisms, such as bacteria, fungi and algae. Complete degradation tends to result in the plastic material being completely transformed into biomass, carbon dioxide and water.
In addition to being biodegradable, the netting must be capable of being made by extrusion. In certain, more common, extruded netting manufacturing processes, plastic netting is typically extruded though an annular die and quenched in a water tank. The resulting tubular netting is collapsed in a quench tank, slit, and opened up to a flat sheet before rewinding. To be compatible with this type of web handling system, the polymer material used in the extruded product must be able to withstand being transformed from an annular tube to a collapsed tube. The material must be flexible enough that this transition does not cause any permanent damage to the netting. The netting's folding point is particularly sensitive to damage (can also affect performance in a subsequent orientation process step). For a given netting material, the greater its thickness, the more susceptible it will be to damage in this transformation process.
PLA polymers are well known biodegradable polymers. However, they are typically high modulus materials. As such, 100% PLA extruded netting, because of its relatively high modulus, does not perform well after extruding in the subsequent web handling and in the orientation portion of the process, thereby making PLA particularly unsuitable for use as an extruded netting material.
Moreover, many extruded biodegradable nettings are subsequently subjected to a process step where it is bi-directionally oriented. To be orientable, the biodegradable material composition must exhibit strain hardening at the orientation temperature, and its stress level in the upper half of the material's strain range must also exceed its yield stress level. In other words, the material must have a break stress higher than the yield stress and exhibit strain between these two stress points. Typically, 100% PLA resins are extrudable, but do not exhibit the above stress-strain relationship, and are therefore not suitable for netting processes that include orientation. For a typical netting extrusion web handling system, the material need to exhibit a certain degree of flexibility, in order for the material not to become damaged by cold stretching when bent or folded.
As such, the material used to make the netting must be able to be extrudable to form netting having desired structural properties, such as flexibility, orientability, tensile strength and biodegradability.